The present invention relates to a photodetector with a multilayer filter, for use in automatic exposure systems for cameras or the like.
FIG. 20 shows a silicon (Si) photodiode conventionally used as a photodetector with a visual sensitivity correction filter for use in automatic exposure systems for cameras. This Si photodiode has the following construction. A light-receiving substrate 2 is mounted on a recessed portion la provided in a ceramic stem 1. A transparent resin is poured into the recessed portion 1a of the stem 1 and then allowed to cure, so that a resin portion 3 with which the light-receiving substrate 2 is covered is formed. A glass absorption filter 10 is attached onto the resin portion 3. Two metallic lead pins 6 are secured through the bottom of the stem 1. Wires made of Au are connected from the metallic lead pins 6 to electrode pads 4, 5 formed on the light-receiving substrate 2. The glass absorption filter 10 is provided with characteristics such as infrared rays cutoff, so that the spectral sensitivity characteristic of the Si photodiode matches the visual sensitivity. Thus, it has been made possible to measure quantity of light in man's visible region during automatic exposure of a camera.
FIG. 21 shows the optical transmittance characteristic of the glass absorption filter 10. For the Si photodiode to be improved in the photo detection sensitivity, the optical transmittance of the glass absorption filter 10 is desirably close to 100%. However, the optical transmittance curve of the glass absorption filter 10 used for the Si photodiode shows a maximum transmittance in the optical transmittance bands of about 80% at the most, as shown in FIG. 21. Therefore, it is difficult to obtain a high photo-detection sensitivity with the glass absorption filter 10. Further, the glass absorption filter 10 has difficulty in design change because its characteristics depend on coloring materials. Accordingly, there is a problem that the spectral sensitivity characteristic of the Si photodiode cannot be easily changed by changing the characteristics of the glass absorption filter 10. In addition, the glass absorption filter 10 is likely to be misted due to moisture in the air or the like, so that there is a question about its reliability.
As a solution for such problems, two of the inventors of the present application have recently designed a photodetector with a multilayer filter which is formed directly on the surface of the light-receiving substrate for the visual sensitivity correction (Japanese Patent Laid-open publication No. 6-77507, issued Mar. 18, 1994). The publication was issued after Applicant's priority date.
FIG. 22 shows a sectional view of the proposed photodetector with a multilayer filter. In FIG. 22, components similar to those of the above-mentioned Si photodiode are designated by the same reference numerals as those used in FIG. 20. In this photodetector with a multilayer filter, instead of the glass absorption filter 10 of the Si photodiode as shown in FIG. 20, an optical multilayer film 20 is formed on the light-receiving substrate 2. The optical multilayer film 20 serves as a filter for adjusting the spectral sensitivity characteristic of the photodetector to the visual sensitivity. Also, the optical multilayer film 20 is formed by alternately laminating low refractive index films made of SiO.sub.2 films and high refractive index films made of TiO.sub.2. These low and high refractive index films are formed directly on the light-receiving substrate 2 by electron-beam deposition or the like.
In order that the films made of TiO.sub.2 have a high refractive index, TiO.sub.2 is deposited on the light-receiving substrate 2 while the light-receiving substrate 2 is kept at a temperature of about 300.degree. C., in usual cases. Then, after the formation of the optical multilayer film 20, parts of the multilayer film on the electrode pads 4, 5 are removed. In this process, because the high refractive index films made of TiO.sub.2 deposited on the light-receiving substrate 2 having a temperature of around 300.degree. C. have been partially crystallized, it would be difficult to remove the multilayer film by wet etching. Therefore, the multilayer film on the electrode pads 4, 5 is removed by a dry etching process such as plasma etching or ion beam etching. The photodetector, which employs the above optical multilayer films 20, is prevented from being misted in the optical multilayer film 20 by humidity and thus free from deterioration in characteristics. Also, since the optical multilayer film 20 serves as a protective film for the light-receiving substrate 2, the photodetector can be prevented from deterioration in characteristics, with its reliability improved.
Unfortunately, the removal of the multilayer film on the electrode pads 4, 5 by dry etching would result in a higher cost than by wet etching. Furthermore, semiconductor crystals of the light-receiving substrate 2 are exposed to ion beams and therefore likely to be damaged.
FIG. 23 shows a sectional view of the light-receiving substrate 2 of the photodetector of FIG. 22. A guard ring electrode 7 is formed on the boundary between a P-type layer 2a and an N-type layer 2b of the light-receiving substrate 2. The guard ring electrode 7, which is formed simultaneously with the electrode pads (not shown in FIG. 23), has such a large degree of surface roughness that the guard ring electrode 7 could not be enough planarized by even laminating a passivation film or a multilayer film thereon. For this reason, part of light 21 incident on the light-receiving substrate 2 strikes the guard ring electrode 7, resulting in scattered light 22. Part of the scattered light 22 becomes light 23 that propagates in the optical multilayer film 20. Then, part of the light 23 reaches an end face 2c of the light-receiving substrate 2 and is scattered there, resulting in scattered light 25. Part of the scattered light 25 enters inside through the end face 2c of the light-receiving substrate 2, whereby light components of undesired wavelengths are received. Meanwhile, another part of the light 23 that propagates in the optical multilayer films 20 is absorbed by the light-receiving substrate 2 during its propagation as indicated by light 24. Further, part of the light incident on the light-receiving substrate 2, although not shown, directly reaches the end face 2c of the light-receiving substrate 2. Such light 24, scattered light 25, and the light that comes directly incident on the end face 2c of the light-receiving substrate 2 may cause the photodetector to deteriorate in its spectral sensitivity characteristic, disadvantageously.